1.3 x 10

ON

FB

SW

L1

C2

R1

R2

R3

LM2695

BST

VCC

D2

C3

C4

D1

ISEN

SGND

LM2695

SNVS413A – JANUARY 2006 – REVISED APRIL 2013

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The start-up regulator is integral to the LM2695. The input pin (VIN) can be connected directly to line voltage up

power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC

pin reaches the under-voltage lockout threshold of 5.7V, the buck switch is enabled and the Softstart pin is

released to allow the Softstart capacitor (C6) to charge up.

(

≊140Ω.

Setting the auxiliary voltage to between 8V and 14V shuts off the internal regulator, reducing internal power

diode connects VCC to VIN. See Figure 6.

Figure 6. Self Biased Configuration

Regulation Comparator

The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V). In normal operation (the output

voltage is regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch

stays on for the programmed on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the

buck switch stays off until the FB voltage falls below 2.5V. Input bias current at the FB pin is less than 100 nA

over temperature.

Over-Voltage Comparator

The voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the on-time

pulse is immediately terminated. This condition can occur if the input voltage or the output load changes

suddenly, or if the inductor (L1) saturates. The buck switch remains off until the voltage at FB falls below 2.5V.

ON-Time Timer, and Shutdown

(5)

8

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